1. Field of the Invention
The present invention relates to process control systems and, more particularly, to automatic detection of oscillation caused by poorly tuned control.
2. Description of the Prior Art
Large complex manufacturing systems, such as pulp and paper mills or chemical petroleum plant, contain often thousands of control loops. The performance of some of these loops is critical to the quality and the uniformity of the final product. In such a large system, variability is introduced in a variety of ways; non-linearity of valves or sensors, controller tuning, process upsets, disturbances, loop interactions, operator interactions and these are only a few of them. Determining what particular problem is affecting the control performance is a non-trivial matter. This can be addressed in two steps: the first step is to detect poorly performing loops and then, in the second step, determine the reason of such poor performance for each loop identified. Or, equivalently, quantification of control loop performance using performance indices, followed by diagnosis to determine what causes the poor performance.
In the article “Assessment of control loop performance”, by T. J. Harris, Can. J. Chem. Eng., 67, pp. 856–861, 1989, Harris proposed comparing the observed level of variance of a controlled variable to the minimum variance controller. In essence, the proposed method detects whether or not there exists an excessive variability; therefore one can categorize loops, into poorly performing loops and acceptable performing loops. The result of the assessment depends on the accuracy of the process time delay, data sampling rate and on how the minimum variance benchmark is estimated. This method does not diagnose directly the cause of the poor performance. Increase in the level of variability is used to detect malfunctioning control loop in U.S. Pat. No. 4,885,676, U.S. Pat. No. 5,249,119 and, more recently in U.S. Pat. No. 6,298,454. The drawback of this approach is that the results are affected by internal process upsets and the lack of diagnostic engine to determine specifically what causes poor performance. The interaction of operators may have a great impact on control loop performance. To resolve this issue, R. Spring in U.S. Pat. No. 5,796,606 proposed to monitor continuously all loops, how often they are in manual mode, and which loops are frequently in alarm. He also proposed to determine recursively certain statistical parameters. The monitoring system resides inside the distributed control system (DCS). This adds more load to the DCS system, and the recursive calculations may be biased due to any loop upset.
In a large industrial plant, poor performance is often due to the large number of loops involved and the lack of personnel available to perform the tuning. Many controllers run with default settings that do not optimize loop performance. In other cases, the tuning is done based on the common sense of operators, which can be a misleading practice. Degradations in control performance due, principally, to tuning problems are not extensively addressed in the literature. Most studies are dedicated to proportional, derivative and integral control loops and how to self tune those parameters as done in WO98/12611. On the other hand, detection of poor performance due to a non-linear phenomenon such as actuator or sensor malfunctioning is the subject of many articles in the literature. In the article “Diagnosis and characterization of oscillations in process control loops”, by A. Horch, A. J. Isaksson and K. Forsman, Control Systems, 161–165, 2000, it is proposed to use the cross-correlation between the control output and the process variable. Their approach states that, in the presence of stiction, the cross-correlation is null at the zero lag. This method addresses only the case of non-linearity caused by stiction. In practice, valves may be subject to stiction or backlash and, more often, to a combination of the two. The problem of actuator failure is addressed in U.S. Pat. No. 5,838,561. In this Patent, the diagnostic of actuator malfunctioning is based on the use of a probability distribution of the deviation error. The reliability of the diagnostic depends on having a large data set, and can be affected by loop upsets. However, this Patent addresses in a very efficient way the problem of poor performance due to loop interactions.